This invention is in the field of generating and transmitting into the earth seismic energy waves. More particularly the invention relates to a controlled-polarization marine seismic shear-wave source having a base adapted to be positioned on a seafloor, i.e., on the floor of a body of water, with controllable azimuthal orientation for predetermining the polarization-orientation of shear waves being generated and propagated into the body of the seafloor material.
Seismic shear waves are transverse elastic waves in which particle motion is perpendicular to the direction of propagation. Horizontally polarized seismic shear waves (SH-Waves) are particularly useful in subsurface exploration and may be generated by subjecting a localized area of a-surface of the earth (seafloor in this instance) to impulses which have a horizontal component of force perpendicular to the direction of propagation of the SH-Waves.
Also, vertically polarized seismic shear waves (SV-Waves) have utility in subsurface exploration and may be generated by subjecting a localized area of a surface of the earth (seafloor) to impulses which have a horizontal component of force in line with the direction of propagation of the SV-Waves.
In addition to generating SV-Waves, subjecting a localized area of a surface of the earth (seafloor) to impulses having a vertical component of force also will generate compressional seismic waves (P-Waves). P-Waves are waves wherein particle motion occurs in the direction of propagation.
It is not possible to transmit seismic shear waves (either SH-Waves or SV-Waves) within a body of water, because water does not provide nor carry shear elastic force. A water body will propagate compressional seismic waves (P-Waves).
SH-Waves, SV-Waves and P-Waves described herein are xe2x80x9cbody wavesxe2x80x9d which propagate within a body of seafloor material and are to be distinguished from xe2x80x9csurface wavesxe2x80x9d which propagate along a surface of the seafloor.
This controlled-polarization marine seismic shear-wave source is capable of generating and propagating down into the seafloor SH-Waves, SV-Waves and P-Waves.
The controlled-polarization marine seismic shear-wave source shown and described herein is adapted to be positioned on a seafloor, i.e., the floor of a body of water, in controlled azimuthal orientation for establishing the polarization-orientation of shear waves being generated and propagated into the body of the seafloor material.
In other words, the azimuthal orientation of the base of the source can be controllably changed by suitable pneumatic thrusters as illustrated and described. Moreover, azimuthal orientation of the base is shown being sensed by a gyroscopic compass for transmitting electrical orientation data signals to a ship by a suitable umbilical cable, or by remote acoustic (sonar) transmission.
Also, azimuthal orientation of the base of the marine seismic shear-wave source can be determined from a ship by using sonar transponders suitably positioned on the base.
The marine seismic shear-wave source embodying the invention includes a suitably robust base adapted to be positioned on a seafloor. This base has downwardly-projecting blade-like teeth elements protruding downwardly therefrom for penetrating into the seafloor surface sediments in mechanical engagement with such seafloor material for augmenting coupling with the seafloor of horizontal force components generated by the shear-wave source.
Mounted on the base is at least one (and preferably two) elongated water-discharge tube(s) aimed upwardly from the base at an acute angle relative to the base. An open discharge mouth is at the upper end of a discharge tube. The open mouth allows water to enter into and fill the tube when the source has been submerged into a body of water. A quick-firing, compressed-air discharger shown as a marine airgun is mounted in a lower portion of the tube. Upon firing a charge of compressed air from this discharger, a slug of water is blasted upwardly along the tube toward its mouth with water surging out of the tube""s mouth, thereby generating a powerful reaction impulse acting in the direction opposite from the direction in which the slug of water is being blasted. This powerful reaction impulse is applied by the base to the seafloor thereby generating SH-Waves and SV-Waves in the seafloor having predetermined polarizations and which propagate in predetermined directions through the body of the seafloor. Also, P-Waves are generated and propagated down into the seafloor body.
In order to control azimuthal orientation of the marine seismic shear-wave source on the seafloor for propagating SH-Waves of predetermined polarization and having a predetermined and known direction of propagation, two pneumatic thrusters are shown mounted on the base of the source. These thrusters are aimed outwardly from the source in opposite directions. They are positioned away from the center of the base so that simultaneous actuation of these two thrusters applies a torque to the base for changing its azimuthal orientation. A preferred way in which to accomplish such change in azimuthal orientation is to use a lifting sling connected to the base of the source. This sling is operated from a ship for raising the base up away from significant engagement with the seafloor. After thus lifting the source, the pneumatic thrusters are actuated for turning the azimuthal orientation of the source while it is supported away from significant contact with the seafloor by the lifting sling.
A swivel is shown provided in a lifting cable line connected to the sling for facilitating turning of azimuthal orientation of the source while it is being supported by the sling and lifting cable.
A gyro compass is shown mounted on the source for providing electric signals to the ship indicating azimuthal orientation of the source. Also, sonar transponders are shown mounted on the base for use in determining azimuthal orientation of the shear-wave source.
In the preferred embodiment of the controlled-polarization marine seismic shear-wave source as shown, its base has a twin pair of elongated water-discharge tubes aimed upwardly and outwardly at equal angles from a vertical centerline.
Thus, a side elevational view of this source is characterized by a symmetrical arrangement of the two water-discharge tubes each extending diagonally upwardly and defining a V-shape between them. The vertical centerline of the source bisects the vertex of this V-shape configuration.
Among advantages of having the twin pair of symmetrically positioned, water-discharge blast tubes are those arising from the capability of alternately discharging them for producing sequences of SH-Waves of alternately opposite polarity.
In addition to the downwardly-projecting blade-teeth elements, the base of the source has a multiplicity of apertures therein for allowing water to flow vertically through the base as it is being lowered through the water suspended by its sling and cable line from a surface ship. These water-flow apertures serve to avoid significant sideways xe2x80x9ckitingxe2x80x9d or xe2x80x9csailingxe2x80x9d through the water as the source is being lowered.
Various other patentably novel features, aspects, advantages and objectives of the controlled-polarization marine seismic shear-wave source embodying the invention will be pointed out and described in the specification with reference to the accompanying drawings and/or will become understood by those skilled in the art from their consideration of the specification and drawings.
The terms xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d are used herein for illustrative convenience in making reference to various Figures. These terms xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d are not intended to be restrictive, because in use the base of the source may be seated down in a seismic exploration or surveying site at a localized region of a seafloor which happens to be inclined relative to a geographically horizontal plane at the site of operation. In such a situation a xe2x80x9cvertical symmetrical centerline CLxe2x80x9d (FIG. 2) of the source will be tipped relative to geographical vertical and the base seated down upon such inclined local region correspondingly will be slanted relative to geographic horizontal.